unfriend her on facebook. stop following her on twitter, snapchat, and your bike. unfollow her friends, unless they truly are mutual friends.
This isn't to spite her. because it's NOT spiting her. it's over. end it. let it end. if you keep it alive, you'll dwell on it. you'll look at her pictures on facebook, you'll see the pics of the parties she goes to, the guys she meets. the guy she moves in with. you'll see pics of their kid. their kid is sitting up now, crawling, walking, omg he's talking now!
but you - you won't move on. and you can't grow without slithering against some rocks and squirming out of your old skin. there's nothing wrong with having a past. she needed you at that time just as you needed her at that time. but neither of you are the same people you once were and you need to man up, grow up, and let her live without you. you won't be happy until you realize that you've both helped each other grow, and now you've outgrown each other. it happens. if she contacts you at some point in the future, great. that doesn't mean take the bait, though. maybe you will no longer have any need of her.